/*
 * Copyright © 2020 pleuvoir (pleuvior@foxmail.com)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.github.pleuvoir.gateway.common.utils;

import java.net.Inet4Address;
import java.net.UnknownHostException;
import java.util.Random;

/**
 * 雪花算法ID生成器
 *
 * @author <a href="mailto:fuwei@daojia-inc.com">pleuvoir</a>
 */
public class IdUtils {

  /**
   * 时间部分所占长度
   */
  private static final int TIME_LEN = 41;
  /**
   * 数据中心id所占长度
   */
  private static final int DATA_LEN = 5;
  /**
   * 机器id所占长度
   */
  private static final int WORK_LEN = 5;
  /**
   * 毫秒内序列所占长度
   */
  private static final int SEQ_LEN = 12;

  /**
   * 定义起始时间 2015-01-01 00:00:00
   */
  private static final long START_TIME = 1420041600000L;
  /**
   * 上次生成ID的时间截
   */
  private static long LAST_TIME_STAMP = -1L;
  /**
   * 时间部分向左移动的位数 22
   */
  private static final int TIME_LEFT_BIT = 64 - 1 - TIME_LEN;

  /**
   * 自动获取数据中心id（可以手动定义 0-31之间的数）
   */
  private static final long DATA_ID = getDataId();
  /**
   * 自动机器id（可以手动定义 0-31之间的数）
   */
  private static final long WORK_ID = getWorkId();
  /**
   * 数据中心id最大值 31
   */
  private static final int DATA_MAX_NUM = ~(-1 << DATA_LEN);
  /**
   * 机器id最大值 31
   */
  private static final int WORK_MAX_NUM = ~(-1 << WORK_LEN);
  /**
   * 随机获取数据中心id的参数 32
   */
  private static final int DATA_RANDOM = DATA_MAX_NUM + 1;
  /**
   * 随机获取机器id的参数 32
   */
  private static final int WORK_RANDOM = WORK_MAX_NUM + 1;
  /**
   * 数据中心id左移位数 17
   */
  private static final int DATA_LEFT_BIT = TIME_LEFT_BIT - DATA_LEN;
  /**
   * 机器id左移位数 12
   */
  private static final int WORK_LEFT_BIT = DATA_LEFT_BIT - WORK_LEN;

  /**
   * 上一次的毫秒内序列值
   */
  private static long LAST_SEQ = 0L;
  /**
   * 毫秒内序列的最大值 4095
   */
  private static final long SEQ_MAX_NUM = ~(-1 << SEQ_LEN);

  public synchronized static long nextId() {
    long now = System.currentTimeMillis();

    // 如果当前时间小于上一次ID生成的时间戳，说明系统时钟回退过这个时候应当抛出异常
    if (now < LAST_TIME_STAMP) {
      throw new RuntimeException(String.format(
          "Clock moved backwards.  Refusing to generate id for %d milliseconds",
          LAST_TIME_STAMP - now));
    }

    if (now == LAST_TIME_STAMP) {
      LAST_SEQ = (LAST_SEQ + 1) & SEQ_MAX_NUM;
      if (LAST_SEQ == 0) {
        now = nextMillis(LAST_TIME_STAMP);
      }
    } else {
      LAST_SEQ = 0;
    }

    // 上次生成ID的时间截
    LAST_TIME_STAMP = now;

    return ((now - START_TIME) << TIME_LEFT_BIT) | (DATA_ID << DATA_LEFT_BIT) | (WORK_ID
        << WORK_LEFT_BIT)
        | LAST_SEQ;
  }

  /**
   * 获取下一不同毫秒的时间戳，不能与最后的时间戳一样
   */
  public static long nextMillis(long lastMillis) {
    long now = System.currentTimeMillis();
    while (now <= lastMillis) {
      now = System.currentTimeMillis();
    }
    return now;
  }

  /**
   * 获取字符串s的字节数组，然后将数组的元素相加，对（max+1）取余
   */
  private static int getHostId(String s, int max) {
    byte[] bytes = s.getBytes();
    int sums = 0;
    for (int b : bytes) {
      sums += b;
    }
    return sums % (max + 1);
  }

  /**
   * 根据 host address 取余，发生异常就获取 0到31之间的随机数
   */
  public static int getWorkId() {
    try {
      return getHostId(Inet4Address.getLocalHost().getHostAddress(), WORK_MAX_NUM);
    } catch (UnknownHostException e) {
      return new Random().nextInt(WORK_RANDOM);
    }
  }

  /**
   * 根据 host name 取余，发生异常就获取 0到31之间的随机数
   */
  public static int getDataId() {
    try {
      return getHostId(Inet4Address.getLocalHost().getHostName(), DATA_MAX_NUM);
    } catch (UnknownHostException e) {
      return new Random().nextInt(DATA_RANDOM);
    }
  }

}